1. Field of the Invention
The present invention relates to methods of detecting and measuring in-situ elastic anisotropy in subterranean rock formations by sensing pressure induced well bore diametral displacements.
2. Description of the Prior Art
A commonly utilized technique for stimulating the production of hydrocarbons from a subterranean rock formation penetrated by a well bore is to create and extend fractures in the formation. Generally, the fractures are created by applying hydraulic pressure on the formation from the well bore. That is, a fracturing fluid is pumped through the well bore and into the formation at a rate and pressure such that the resultant hydraulic force exerted on the formation causes one or more fractures to be created therein. The fractures are extended by continued pumping, and the fractures can be propped open or flow channels can be etched in the faces of the fractures with acid or both to provide openings in the formation through which hydrocarbons readily flow to the well bore. Fracturing is also utilized in carrying out enhanced production procedures in subterranean formations as well as in other applications.
In designing fracturing treatments to be carried out in subterranean rock formations, it is often necessary and always desirable to know the direction in which fractures will extend in the formation and other directional fracture related characteristics such as in-situ rock elastic moduli, in-situ stresses, etc. Heretofore, the fracture direction and other subterranean rock formation characteristics have been determined or attempted to be determined by analyzing core samples from the formation. For example, U.S. Pat. No. 4,529,036 issued Jul. 16, 1985 to Daneshy et al. discloses a method of determining the orientation of a fracture or fractures created in a subterranean formation. In accordance with that method, the formation is hydraulically fractured at the lower end portion of the well bore and an azimuthally oriented core containing a portion of the fracture is removed from beneath the bottom of the well bore. An inspection of the core coupled with a knowledge of its orientation in the well bore are used to determine the direction of hydraulically induced fractures in the formation. While the method of Daneshy et al. has been utilized successfully for determining fracture direction, it is relatively time consuming and expensive as a result of the necessity of removing and testing a core, it does not provide other fracture related characteristics of the formation such as those described above and fracturing information is only obtainable at the conclusion of the test. Further, if the fracturing procedure is unsuccessful, the coring operation and the testing of the core are performed without knowledge of whether the core does or does not contain a fracture.
More recently, tools have been developed for measuring the in-situ enlargements of a well bore penetrating a subterranean formation in response to pressure exerted on the formation. Such a tool is described in U.S. Pat. No. 4,673,890 issued Jun. 16, 1987 to Copland et al. In the use of the tool, it is connected to a string of pipe and lowered in the well bore to a point adjacent a particular subterranean formation. The tool is isolated and locked in the well bore and increasing pressure is applied to the formation to a pressure level whereby the rock formation adjacent the tool fractures. As the pressure is being increased, the tool measures incremental diametral displacements of the well bore which are processed and recorded. The tool and the measurements are azimuthally oriented and the measurements are utilized to determine the direction of the fracture or fractures created in the formation.
By the present invention, a method is provided for using a tool such as the tool described in U.S. Pat. No. 4,673,890 to detect and measure in-situ elastic anisotropy in a subterranean rock formation in addition to determining fracture direction and fracture width as a function of time and pressure. The detection and measurement of elastic anisotropy allows the calculation of directional in-situ rock elastic moduli, the comparison of anisotropy to current in-situ stress direction and the investigation of potential anelastic formation anisotropies through pressure cycling. A comparison of the principal directions of the in-situ moduli with those of the in-situ stresses found from hydraulic fracture direction can provide insight into the history of the stress field. Such information is used for designing subsequent fracture treatments, for making realistic and accurate fracture models and for aiding in the understanding of the geology, geophysical characteristics and/or stress orientations of a region.